Question: There are 9 trees on the right bank of a river. There are 20 trees on the left bank. How many trees are on the banks of the river?
Answer: The total number of trees on the two banks of the river is the sum of the number of trees on the left bank and the number of trees on the right bank. The sum is $9 + 20$ trees. $9 + 20 = 29$ trees.